The present invention relates to a system for efficiently exchanging heat with ground water in an aquifer and more particularly to a system which may be installed in a single well bore and which by selectively creating a temperature difference within the aquifer produces convective effects that greatly increase the usefulness of the aquifer as both a heat source and a heat sink for exchanging heat with a utilization device, such as a heat pump, at the earth's surface.
With the growing search for alternative energy sources, new attention has been turned to extracting heat from within the earth. For many years different systems have been proposed, and some actually utilized, for withdrawing geothermal heat from regions deep beneath the earth's surface using water as the heat-exchange medium. An example of one system of this type is taught in U.S. Pat. No. 2,461,499 to Smith et al wherein a heat pump at the earth's surface is supplied with water pumped through two pipes extending down a single bore deep well, which pipes respectively withdraw and return water at different temperatures from and to aquifers in geothermally heated regions within the earth. This system takes advantage of the geothermal gradient, that is, the increase in temperature within the earth of about 1.degree. F. for each 50 feet of depth, so that the pipes extend into regions that are 200 to 300 feet or much farther below the earth's surface. Since the geothermal temperature gradient is the opposite of that normall occurring in water, the cool water is located above the warm water in such deep regions. Consequently, the lower ends of the pipes are separated by at least 100 feet or more to avoid local convective mixing of the supply and return water in the aquifer, and a well packer is used therebetween to avoid mixing within the well. Accordingly, in such systems, the water, heated by the geothermally heated ambient rock at great depths, is brought to the earth's surface from the lower regions as a heat source, exchanges its heat at the surface and after cooling is exhausted to an upper region.
Other systems are known for drawing water from shallower aquifers at distances of 20 to 150 feet below the earth's surface for heat exchange purposes. A review of these latter systems, using a heat pump at the earth's surface, is presented in the September, 1980 issue of POPULAR MECHANICS magazine in an article at pages 155 to 158 entitled "How to Tap the Energy Under Your Backyard" by John H. Ingersoll. In contrast to the geothermally heated water in deep wells, shallower aquifers contain ground water which runs off from the earth's surface and primarily contains solar heat at relatively lower temperatures. Also, while geothermally heated water is at different temperatures at different subterranean levels, with the temperature increasing with depth, ground water is found to be of a comparatively uniform temperature and remains at a fairly constant year-round temperature in the aquifer equal to the mean annual temperature of its given geographical region. Consequently, the easily accessible ground water may act as a source of heat in winter by passing it through a suitable heat exchanger in contact with the relatively cooler atmosphere and as a heat sink in summer by passing it through a heat exchanger in contact with the relatively warmer atmosphere. However, once the heat has been exchanged with the water at the earth's surface, a problem is then presented by the need to dispose of the used or back water to provide a continuous flow for effective heat exchange.
As indicated in the cited POPULAR MECHANICS article, the two common solutions to the back water problem are dumping the water into a discharge pond or returning it down a second or recharge well directly into the same aquifer from which it has been withdrawn through a separate supply well. This practice is confirmed in the May, 1982 issue of POPULAR SCIENCE magazine at page 66 in an article entitled "Heat-Pump Water Heaters" by Evan Powell. In most locations the use or provision of a suitable discharge pond or stream, in view of the volume of water which may be discharged, will require not only proper soil type and grading, but possibly considerable piping and excavation. In addition, the dumping of such water in the wintertime when freezing may occur will usually require further modifications and adaptations of the discharge system. Further, if used on a large scale, such systems risk the depleting of the aquifer and exhausting of the sources of potable water. As a result, it appears that the preferred discharge system is a recharge well down which the back water may be returned to the aquifer. However, to avoid mixing of or heat exchange between the supply water and the back water in the aquifer, the recharge well must be a considerable distance from the supply well, so that in addition to the expense of drilling a second well, the expense of extensive piping and excavation may also be necessary. A further problem presented by the use of a recharge well is the fact that in many states of the United States, these wells are either not permitted or require special permits. An example of a system using supply and recharge wells is found in U.S. Pat. No. 2,637,531 to Davidson.
It would, of course, be desirable to withdraw and return the water using a single well bore, such as in the previously-mentioned Smith et al system, but as noted, the conditions in a shallow aquifer are different from those in the deep wells contemplated for use by Smith et al. As indicated above, in these deep wells the water is withdrawn from a region which is at one temperature and returned to a region which is at a different temperature, the cooler regions being disposed above the warmer regions in different aquifers, or in the same aquifer if it is of considerable depth. Since convective forces will normally cause the greater density cool water to seek a level below the lesser density warm water, the frictional characteristics of such aquifers, that is, their resistance to convective flow, must be relied upon to avoid mixing and heat exchange of the supply and return or back water. Reliance on this resistance is the reason for the large separation between the pipe ends in the Smith et al system. In contrast, within shallower ground water aquifers the physical characteristics are such that a uniform temperature is present in the aquifer, so that the use of the Smith et al system therein would not normally be feasible since, were such a system used in the aquifer, it would promote convective mixing of upper and lower layers thereby rapidly raising or lowering the temperature of the water at the inlet pipe and vitiating its usefulness for surface heat exchange. It has accordingly been the experience of those skilled in the art that if the back water is simply returned to the same well bore, either as in Smith et al or randomly, the back water will mix with the supply water being drawn from the aquifer by virtue of both the mechanical action of the pump and the convective forces within the bore and aquifer, thus quickly negating the heat exchange value of the supply water.
The present invention is directed to a system for use with shallower ground water aquifers that solves the back water problem and wherein a single well bore is used as both the supply well and recharge well for circulating ground water through heat-exchanging apparatus at the earth's surface, thus obviating the need for extensive piping, drilling or excavation as in the prior art systems of this type while achieving comparable or better heat exchange results and avoiding aquifer depletion and potable water loss. In addition, an alternative embodiment to the preferred embodiment is provided for significantly reducing the amount of input energy necessary to operate the system by obviating the need for pumping the water from the aquifer. A modification by which both embodiments may be adapted to permit the storage of further solar energy collected at the earth's surface in the aquifer is also described as well as a valved plenum arrangement for particular use with the former embodiment and a low-energy consumption adaptation of the former embodiment.